In radio communication systems designed to service a large area, it is often desirable to provide a large number of receivers, each positioned at a different location within the area to be serviced. Generally, the receivers may be base stations, repeaters or other communication units capable of receiving signals. The base stations or repeaters usually have an outbound signal power much greater than the inbound signal power of users' mobile or portable communication units and can enable fringe users to communicate with other users' communication units and/or receivers that otherwise might not be accessible under their own power. Typically, the receivers have overlapping effective reception ranges so that messages transmitted within the area may be picked up by any of several receivers on the same frequency or channel. As an example of such a system, a communication unit transmits via a radio frequency (RF) communication resource a message that is received by a number of remote receivers. Each of the receivers sends its received signal to a comparator, also known as a voter typically by wireline communications. The comparator receives and compares each message from each of the receivers and outputs a message that is comprised of either an entire message from one of the receivers or a composite message comprised of segments of the message received from one or more of the receivers. Each message may be comprised of a plurality of message frames.
Generally, comparators include a fixed number of communication ports for receiving and/or transmitting messages and a processor for executing voting functions. For example, ASTROTAC.TM. or ASTROTAC 3000.TM. comparators, available from Motorola, Inc., have sixteen communication ports that may be used to receive up to sixteen messages. Occasionally, the number of receivers in the system might exceed the capacity of an individual comparator chassis to receive messages (e.g., for the ASTROTAC 3000.TM. comparator, greater than sixteen messages), in which case one or more secondary comparator chassis may be connected to a primary comparator chassis to form an "expanded comparator." Preferably, the expanded comparator operates to vote among the several messages and produce an output message generally as described above.
There are a variety of voting methods that have been devised for use in comparators. In one method, known as a single-stage method, all of the message frames received by the secondary comparators are forwarded to the primary comparator for voting. One of the problems associated with this method is that the number of messages received by the primary comparator may exceed the amount that may be effectively voted, due to processing and/or hardware limitations in the primary comparator and/or communication link. Also known are cascading methods in which messages received at a first comparator are voted to yield a first voted message that is forwarded to a second comparator; messages received at the second comparator (including the first voted message and various "unvoted" messages assigned to the second comparator) are voted to yield a second voted message, and so on until reaching a final-stage comparator that yields a final voted signal. This method has been known to exhibit large voting delays as each consecutive chassis must wait to receive voted message(s) from the previous chassis in addition to its assigned "unvoted" messages.
Accordingly, there is a need for a comparator and voting methods for use in the comparator that produce a voted signal without significant voting delays and without exceeding processing and/or hardware limitations of any particular components of the comparator. The present invention is directed to satisfying or at least partially satisfying these needs.